Transmissions of the mechanical type, the hydraulic type (so-called HST), the hydro-mechanical type, the belt type, and the like are generally known. Further, the types are classified, respectively. For example, the mechanical type is classified into the sliding-mesh type, the constant-mesh type, the synchromesh type, the planetary gear type, and the like.
Among the various transmissions, the hydro-mechanical transmission has a hydraulic power transmitting mechanism operated by a hydraulic pump and a hydraulic motor and a mechanical power transmitting mechanism using gears, which are switchably used. The speed of input rotation is changed and resultant rotation can be generated. Hydro-mechanical transmissions, as shown below, are recently known, in which the hydraulic power transmitting mechanism, by which a high torque can be obtained, is used at the time of low-speed output and the mechanical power transmitting mechanism, by which high transmission efficiency can be obtained, is used at the time of high-speed output.
(1) According to a hydro-mechanical transmission disclosed in Japanese Patent Application Laid-Open No. 5-44815 (a prior proposal by the applicant of the present invention), two planetary gear mechanisms are commonly used by a mechanical power transmitting mechanism and a hydraulic power transmitting mechanism and four directly-coupled clutches are used, thereby enabling each of forward and reverse hydraulic travel at the 1st to 3rd speeds and enabling each of the forward and reverse mechanical travel at the 1st to 3rd speeds to be switched.
(2) According to a hydro-mechanical transmission disclosed in Japanese Patent Application Laid-Open No. 6-159474, four counter-shaft clutches are commonly used by the mechanical and hydraulic power transmitting mechanisms and two directly-coupled clutches are used; so that reverse hydraulic travel at the 1st speed, in the event of deadheading a vehicle; forward starting hydraulic travel and forward mechanical travel at the 1st to 4th speeds; and forward hydraulic travel at the 1st to 4th speeds, in the event of travel for work, can be switched.
A mobile vehicle having a hydraulically driven working machine, for example, a wheeled hydraulic excavator, has a large number of opportunities to fully drive the working machine while the vehicle drives at low speed. It is also desirable that it move at high speed on highways. When the hydro-mechanical transmission is mounted on such a mobile vehicle, the forward/reverse travel can be efficiently switched by using the hydraulic power transmitting mechanism at the time of low speed travel. What is more, the hydraulic power transmitting mechanism has a variable displacement hydraulic pump and motor like the HST. There are also advantages that the vehicle can start with a high torque, perform the non-stage transmission, and distribute a large hydraulic power for the working machine irrespective of the driving speed. On the other hand, when the vehicle drives at high speed, there is also an advantage that the travel with high transmission efficiency can be realized by using the mechanical power transmitting mechanism.
With respect to the conventional hydro-mechanical transmissions, however, whether the hydraulic pump and the hydraulic motor of the hydraulic power transmitting mechanism are of a variable displacement type or not is not written. When it is assumed that the hydraulic pump and motor are of the variable displacement type, since the speed can be changed in a non-stage manner, the purpose of the description of the examples that "gears are shifted at a multi-stage by two planetary gear apparatus and four counter-shaft clutches" is unclear. In the examples, although a number of directly-coupled clutches are used so that the hydraulic travel in which the speed is changed at the multi-stage and the mechanical travel in which the speed is changed at the multi-stage can be switched, this makes the construction complicated. It cannot be said that the conventional techniques use most of the above-mentioned advantages of the hydro-mechanical transmission.